Galectin-9 binds to O-glycans on protein disulfide isomerase.

Changes in the T cell surface redox environment regulate critical cell functions, such as cell migration, viral entry and cytokine production. Cell surface protein disulfide isomerase (PDI) contributes to the regulation of T cell surface redox status. Cell surface PDI can be released into the extracellular milieu or can be internalized by T cells. We have found that galectin-9, a soluble lectin expressed by T cells, endothelial cells and dendritic cells, binds to and retains PDI on the cell surface. While endogenous galectin-9 is not required for basal cell surface PDI expression, exogenous galectin-9 mediated retention of cell surface PDI shifted the disulfide/thiol equilibrium on the T cell surface. O-glycans on PDI are required for galectin-9 binding, and PDI recognition appears to be specific for galectin-9, as galectin-1 and galectin-3 do not bind PDI. Galectin-9 is widely expressed by immune and endothelial cells in inflamed tissues, suggesting that T cells would be exposed to abundant galectin-9, in cis and in trans, in infectious or autoimmune conditions.

Differentiation-related glycan epitopes identify discrete domains of the muscle glycocalyx.

The neuromuscular junction (NMJ) is enriched with glycoproteins modified with N-acetylgalactosamine (GalNAc) residues, and four nominally GalNAc-specific plant lectins have historically been used to identify the NMJ and the utrophin-glycoprotein complex. However, little is known about the specific glycan epitopes on skeletal muscle that are bound by these lectins, the glycoproteins that bear these epitopes or how creation of these glycan epitopes is regulated. Here, we profile changes in cell surface glycosylation during muscle cell differentiation and identify distinct differences in the binding preferences of GalNAc-specific lectins, Wisteria floribunda agglutinin (WFA), Vicia villosa agglutinin (VVA), soybean agglutinin (SBA) and Dolichos biflorus agglutinin (DBA). While we find that all four GalNAc binding lectins specifically label the NMJ, each of the four lectins binds distinct sets of muscle glycoproteins; furthermore, none of the major adhesion complexes are required for binding of any of the four GalNAc-specific lectins. Analysis of glycosylation-related transcripts identified target glycosyltransferases and glycosidases that could potentially create GalNAc-containing epitopes; reducing expression of these transcripts by siRNA highlighted differences in lectin binding specificities. In addition, we found that complex N-glycans are required for binding of WFA and SBA to murine C2C12 myotubes and for WFA binding to wild-type skeletal muscle, but not for binding of VVA or DBA. These results demonstrate that muscle cell surface glycosylation is finely regulated during muscle differentiation in a domain- and acceptor-substrate-specific manner, suggesting that temporal- and site-specific glycosylation are important for skeletal muscle cell function.

Galectin-3 binds to CD45 on diffuse large B-cell lymphoma cells to regulate susceptibility to cell death.

Diffuse large B-cell lymphoma (DLBCL) is the most common non-Hodgkin lymphoma and an aggressive malignancy. Galectin-3 (gal-3), the only antiapoptotic member of the galectin family, is overexpressed in DLBCL. While gal-3 can localize to intracellular sites, gal-3 is secreted by DLBCL cells and binds back to the cell surface in a carbohydrate-dependent manner. The major counterreceptor for gal-3 on DLBCL cells was identified as the transmembrane tyrosine phosphatase CD45. Removal of cell-surface gal-3 from CD45 with the polyvalent glycan inhibitor GCS-100 rendered DLBCL cells susceptible to chemotherapeutic agents. Binding of gal-3 to CD45 modulated tyrosine phosphatase activity; removal of endogenous cell-surface gal-3 from CD45 with GCS-100 increased phosphatase activity, while addition of exogenous gal-3 reduced phosphatase activity. Moreover, the increased susceptibility of DLBCL cells to chemotherapeutic agents after removal of gal-3 by GCS-100 required CD45 phosphatase activity. Gal-3 binding to a subset of highly glycosylated CD45 glycoforms was regulated by the C2GnT-1 glycosyltransferase, indicating that specific glycosylation of CD45 is important for regulation of gal-3-mediated signaling. These data identify a novel role for cell-surface gal-3 and CD45 in DLBCL survival and suggest novel therapeutic targets to sensitize DLBCL cells to death.

High throughput screening for compounds that alter muscle cell glycosylation identifies new role for N-glycans in regulating sarcolemmal protein abundance and laminin binding.

Duchenne muscular dystrophy is an X-linked disorder characterized by loss of dystrophin, a cytoskeletal protein that connects the actin cytoskeleton in skeletal muscle cells to extracellular matrix. Dystrophin binds to the cytoplasmic domain of the transmembrane glycoprotein ß-dystroglycan (ß-DG), which associates with cell surface α-dystroglycan (α-DG) that binds laminin in the extracellular matrix. ß-DG can also associate with utrophin, and this differential association correlates with specific glycosylation changes on α-DG. Genetic modification of α-DG glycosylation can promote utrophin binding and rescue dystrophic phenotypes in mouse dystrophy models. We used high throughput screening with the plant lectin Wisteria floribunda agglutinin (WFA) to identify compounds that altered muscle cell surface glycosylation, with the goal of finding compounds that increase abundance of α-DG and associated sarcolemmal glycoproteins, increase utrophin usage, and increase laminin binding. We identified one compound, lobeline, from the Prestwick library of Food and Drug Administration-approved compounds that fulfilled these criteria, increasing WFA binding to C2C12 cells and to primary muscle cells from wild type and mdx mice. WFA binding and enhancement by lobeline required complex N-glycans but not O-mannose glycans that bind laminin. However, inhibiting complex N-glycan processing reduced laminin binding to muscle cell glycoproteins, although O-mannosylation was intact. Glycan analysis demonstrated a general increase in N-glycans on lobeline-treated cells rather than specific alterations in cell surface glycosylation, consistent with increased abundance of multiple sarcolemmal glycoproteins. This demonstrates the feasibility of high throughput screening with plant lectins to identify compounds that alter muscle cell glycosylation and identifies a novel role for N-glycans in regulating muscle cell function.

Structural aspects of binding of α-linked digalactosides to human galectin-1.

By definition, adhesion/growth-regulatory galectins are known for their ability to bind ß-galactosides such as Galß(1 → 4)Glc (lactose). Indications for affinity of human galectin-1 to α-linked digalactosides pose questions on the interaction profile with such bound ligands and selection of the galactose moiety for CH-π stacking. These issues are resolved by a combination of (15)N-(1)H heteronuclear single quantum coherence (HSQC) chemical shift and saturation transfer difference nuclear magnetic resonance (STD NMR) epitope mappings with docking analysis, using the α(1 → 3/4)-linked digalactosides and also Galα(1 → 6)Glc (melibiose) as test compounds. The experimental part revealed interaction with the canonical lectin site, and this preferentially via the non-reducing-end galactose moiety. Low-energy conformers appear to be selected without notable distortion, as shown by molecular dynamics simulations. With the α(1 → 4) disaccharide, however, the typical CH-π interaction is significantly diminished, yet binding appears to be partially compensated for by hydrogen bonding. Overall, these findings reveal that the type of α-linkage in digalactosides has an impact on maintaining CH-π interactions and the pattern of hydrogen bonding, explaining preference for the α(1 → 3) linkage. Thus, this lectin is able to accommodate both α- and ß-linked galactosides at the same site, with major contacts to the non-reducing-end sugar unit.

Lactose binding to galectin-1 modulates structural dynamics, increases conformational entropy, and occurs with apparent negative cooperativity.

Galectins are a family of lectins with a conserved carbohydrate recognition domain that interacts with beta-galactosides. By binding cell surface glycoconjugates, galectin-1 (gal-1) is involved in cell adhesion and migration processes and is an important regulator of tumor angiogenesis. Here, we used heteronuclear NMR spectroscopy and molecular modeling to investigate lactose binding to gal-1 and to derive solution NMR structures of gal-1 in the lactose-bound and unbound states. Structure analysis shows that the beta-strands and loops around the lactose binding site, which are more open and dynamic in the unbound state, fold in around the bound lactose molecule, dampening internal motions at that site and increasing motions elsewhere throughout the protein to contribute entropically to the binding free energy. CD data support the view of an overall more open structure in the lactose-bound state. Analysis of heteronuclear single quantum coherence titration binding data indicates that lactose binds the two carbohydrate recognition domains of the gal-1 dimer with negative cooperativity, in that the first lactose molecule binds more strongly (K(1)=21+/-6 x 10(3) M(-1)) than the second (K(2)=4+/-2 x 10(3) M(-1)). Isothermal calorimetry data fit using a sequential binding model present a similar picture, yielding K(1)=20+/-10 x 10(3) M(-1) and K(2)=1.67+/-0.07 x 10(3) M(-1). Molecular dynamics simulations provide insight into structural dynamics of the half-loaded lactose state and, together with NMR data, suggest that lactose binding at one site transmits a signal through the beta-sandwich and loops to the second binding site. Overall, our results provide new insight into gal-1 structure-function relationships and to protein-carbohydrate interactions in general.

Galectin-1 co-clusters CD43/CD45 on dendritic cells and induces cell activation and migration through Syk and protein kinase C signaling.

Galectin-1 is a galactoside-binding lectin expressed in multiple tissues that has pleiotropic immunomodulatory functions. We previously showed that galectin-1 activates human monocyte-derived dendritic cells (MDDCs) and triggers a specific genetic program that up-regulates DC migration through the extracellular matrix, an integral property of mucosal DCs. Here, we identify the galectin-1 receptors on MDDCs and immediate downstream effectors of galectin-1-induced MDDC activation and migration. Galectin-1 binding to surface CD43 and CD45 on MDDCs induced an unusual unipolar co-clustering of these receptors and activates a dose-dependent calcium flux that is abrogated by lactose. Using a kinome screen and a systems biology approach, we identified Syk and protein kinase C tyrosine kinases as mediators of the DC activation effects of galectin-1. Galectin-1, but not lipopolysaccharide, stimulated Syk phosphorylation and recruitment of phosphorylated Syk to the CD43 and CD45 co-cluster on MDDCs. Inhibitors of Syk and protein kinase C signaling abrogated galectin-1-induced DC activation as monitored by interleukin-6 production; and MMP-1, -10, and -12 gene up-regulation; and enhanced migration through the extracellular matrix. The latter two are specific features of galectin-1-activated DCs. Interestingly, we also found that galectin-1 can prime DCs to respond more quickly to low dose lipopolysaccharide stimulation. Finally, we underscore the biological relevance of galectin-1-enhanced DC migration by showing that intradermal injection of galectin-1 in MRL-fas mice, which have a defect in skin DC emigration, increased the in vivo migration of dermal DCs to draining lymph nodes.

CD45-mediated fodrin cleavage during galectin-1 T cell death promotes phagocytic clearance of dying cells.

Disassembly and phagocytic removal of dying cells is critical to maintain immune homeostasis. The factors that regulate fragmentation and uptake of dying lymphocytes are not well understood. Degradation of fodrin, a cytoskeletal linker molecule that attaches CD45 to the actin cytoskeleton, has been described in apoptotic cells, although no specific initiator of fodrin degradation has been identified. CD45 is a glycoprotein receptor for galectin-1, an endogenous lectin that can trigger lymphocyte apoptosis, although CD45 is not required for phosphatidylserine externalization or DNA degradation during galectin-1 death. In this study, we show that fodrin degradation occurs during galectin-1 T cell death and that CD45 is essential for fodrin degradation to occur. In the absence of CD45, or if fodrin degradation is prevented, galectin-1-induced cell death is not accompanied by membrane blebbing, although phosphatidylserine externalization and DNA degradation proceed, indicating that fodrin degradation occurs via a distinct pathway compared with the pathway that leads to these other hallmarks of cell death. Moreover, there is slower phagocytic uptake by macrophages of T cells in which fodrin degradation is prevented, relative to T cells in which CD45-mediated fodrin degradation occurs. These studies identify a novel role for CD45 in regulating cellular disassembly and promoting phagocytic clearance during galectin-1-induced T cell death.

Suppression of autoimmune diabetes by soluble galectin-1.

Type 1 diabetes (T1D) is a T cell-mediated autoimmune disease that targets the beta-cells of the pancreas. We investigated the ability of soluble galectin-1 (gal-1), an endogenous lectin that promotes T cell apoptosis, to down-regulate the T cell response that destroys the pancreatic beta-cells. We demonstrated that in nonobese diabetic (NOD) mice, gal-1 therapy reduces significantly the amount of Th1 cells, augments the number of T cells secreting IL-4 or IL-10 specific for islet cell Ag, and causes peripheral deletion of beta-cell-reactive T cells. Administration of gal-1 prevented the onset of hyperglycemia in NOD mice at early and subclinical stages of T1D. Preventive gal-1 therapy shifted the composition of the insulitis into an infiltrate that did not invade the islets and that contained a significantly reduced number of Th1 cells and a higher percentage of CD4(+) T cells with content of IL-4, IL-5, or IL-10. The beneficial effects of gal-1 correlated with the ability of the lectin to trigger apoptosis of the T cell subsets that cause beta-cell damage while sparing naive T cells, Th2 lymphocytes, and regulatory T cells in NOD mice. Importantly, gal-1 reversed beta-cell autoimmunity and hyperglycemia in NOD mice with ongoing T1D. Because gal-1 therapy did not cause major side effects or beta-cell toxicity in NOD mice, the use of gal-1 to control beta-cell autoimmunity represents a novel alternative for treatment of subclinical or ongoing T1D.

Endogenous galectin-1 enforces class I-restricted TCR functional fate decisions in thymocytes.

During thymocyte development, the T-cell receptor (TCR) can discriminate major histocompatibility complex (MHC)/peptide ligands over a narrow range of affinities and translate subtle differences into functional fate decisions. How small differences in TCR input are translated into absolute differences in functional output is unclear. We examined the effects of galectin-1 ablation in the context of class-I-restricted thymocyte development. Galectin-1 expression opposed TCR partial agonist-driven positive selection, but promoted TCR agonist-driven negative selection of conventional CD8(+) T cells. Galectin-1 expression also promoted TCR agonist-driven CD8alphaalpha intestinal intraepithelial lymphocytes (IEL) development. Recombinant galectin-1 enhanced TCR binding to agonist/MHC complexes and promoted a negative-selection-signaling signature, reflected in intensified rapid and transient extracellular signal-regulated kinase (ERK) activation. In contrast, galectin-1 expression antagonized ERK activity in thymocytes undergoing positive selection. We propose that galectin-1 aids in discriminating TCR-directed fate decisions by promoting TCR binding to agonist/MHC complexes and enforcing agonist-driven signals, while opposing partial-agonist signals. In this way, galectin-1 widens the distinction between TCR-directed functional fate cues.

1H, 13C, and 15N backbone and side-chain chemical shift assignments for the 29 kDa human galectin-1 protein dimer.

Galectin-1 is an important regulator of leukocyte function and tumor angiogenesis. Recently, this lectin has been identified as a molecular target for the potent angiogenesis inhibitor anginex. Here, we report (1)H, (13)C, and (15)N chemical shift assignments for human galectin-1 as determined by using heteronuclear triple resonance NMR spectroscopy.

Galectin-1-matured human monocyte-derived dendritic cells have enhanced migration through extracellular matrix.

Dendritic cells (DCs) are potent mediators of the immune response, and can be activated by exogenous pathogen components. Galectin-1 is a member of the conserved beta-galactoside-binding lectin family that binds galactoside residues on cell surface glycoconjugates. Galectin-1 is known to play a role in immune regulation via action on multiple immune cells. However, its effects on human DCs are unknown. In this study, we show that galectin-1 induces a phenotypic and functional maturation in human monocyte-derived DCs (MDDCs) similar to but distinct from the activity of the exogenous pathogen stimuli, LPS. Immature human MDDCs exposed to galectin-1 up-regulated cell surface markers characteristic of DC maturation (CD40, CD83, CD86, and HLA-DR), secreted high levels of IL-6 and TNF-alpha, stimulated T cell proliferation, and showed reduced endocytic capacity, similar to LPS-matured MDDCs. However, unlike LPS-matured DCs, galectin-1-treated MDDCs did not produce the Th1-polarizing cytokine IL-12. Microarray analysis revealed that in addition to modulating many of the same DC maturation genes as LPS, galectin-1 also uniquely up-regulated a significant subset of genes related to cell migration through the extracellular matrix (ECM). Indeed, compared with LPS, galectin-1-treated human MDDCs exhibited significantly better chemotactic migration through Matrigel, an in vitro ECM model. Our findings show that galectin-1 is a novel endogenous activator of human MDDCs that up-regulates a significant subset of genes distinct from those regulated by a model exogenous stimulus (LPS). One unique effect of galectin-1 is to increase DC migration through the ECM, suggesting that galectin-1 may be an important component in initiating an immune response.

Galectin-3 and galectin-1 bind distinct cell surface glycoprotein receptors to induce T cell death.

Galectins are a family of mammalian beta-galactoside-binding proteins that positively and negatively regulate T cell death. Extracellular galectin-1 directly induces death of T cells and thymocytes, while intracellular galectin-3 blocks T cell death. In contrast to the antiapoptotic function of intracellular galectin-3, we demonstrate that extracellular galectin-3 directly induces death of human thymocytes and T cells. However, events in galectin-3- and galectin-1-induced cell death differ in a number of ways. Thymocyte subsets demonstrate different susceptibility to the two galectins: whereas galectin-1 kills double-negative and double-positive human thymocytes with equal efficiency, galectin-3 preferentially kills double-negative thymocytes. Galectin-3 binds to a complement of T cell surface glycoprotein receptors distinct from that recognized by galectin-1. Of these glycoprotein receptors, CD45 and CD71, but not CD29 and CD43, appear to be involved in galectin-3-induced T cell death. In addition, CD7 that is required for galectin-1-induced death is not required for death triggered by galectin-3. Following galectin-3 binding, CD45 remains uniformly distributed on the cell surface, in contrast to the CD45 clustering induced by galectin-1. Thus, extracellular galectin-3 and galectin-1 induce death of T cells through distinct cell surface events. However, as galectin-3 and galectin-1 cell death are neither additive nor synergistic, the two death pathways may converge inside the cell.

Novel innate immune functions for galectin-1: galectin-1 inhibits cell fusion by Nipah virus envelope glycoproteins and augments dendritic cell secretion of proinflammatory cytokines.

Galectin-1 (gal-1), an endogenous lectin secreted by a variety of cell types, has pleiotropic immunomodulatory functions, including regulation of lymphocyte survival and cytokine secretion in autoimmune, transplant disease, and parasitic infection models. However, the role of gal-1 in viral infections is unknown. Nipah virus (NiV) is an emerging pathogen that causes severe, often fatal, febrile encephalitis. The primary targets of NiV are endothelial cells. NiV infection of endothelial cells results in cell-cell fusion and syncytia formation triggered by the fusion (F) and attachment (G) envelope glycoproteins of NiV that bear glycan structures recognized by gal-1. In the present study, we report that NiV envelope-mediated cell-cell fusion is blocked by gal-1. This inhibition is specific to the Paramyxoviridae family because gal-1 did not inhibit fusion triggered by envelope glycoproteins of other viruses, including two retroviruses and a pox virus, but inhibited fusion triggered by envelope glycoproteins of the related Hendra virus and another paramyxovirus. The physiologic dimeric form of gal-1 is required for fusion inhibition because a monomeric gal-1 mutant had no inhibitory effect on cell fusion. gal-1 binds to specific N-glycans on NiV glycoproteins and aberrantly oligomerizes NiV-F and NiV-G, indicating a mechanism for fusion inhibition. gal-1 also increases dendritic cell production of proinflammatory cytokines such as IL-6, known to be protective in the setting of other viral diseases such as Ebola infections. Thus, gal-1 may have direct antiviral effects and may also augment the innate immune response against this emerging pathogen.

Theta defensins protect cells from infection by herpes simplex virus by inhibiting viral adhesion and entry.

We tested the ability of 20 synthetic theta defensins to protect cells from infection by type 1 and type 2 herpes simplex viruses (HSV-1 and -2, respectively). The peptides included rhesus theta defensins (RTDs) 1 to 3, originally isolated from rhesus macaque leukocytes, and three peptides (retrocyclins 1 to 3) whose sequences were inferred from human theta-defensin (DEFT) pseudogenes. We also tested 14 retrocyclin analogues, including the retro, enantio, and retroenantio forms of retrocyclin 1. Retrocyclins 1 and 2 and RTD 3 protected cervical epithelial cells from infection by both HSV serotypes, but only retrocyclin 2 did so without causing cytotoxicity or requiring preincubation with the virus. Surface plasmon resonance studies revealed that retrocyclin 2 bound to immobilized HSV-2 glycoprotein B (gB2) with high affinity (K(d), 13.3 nM) and that it did not bind to enzymatically deglycosylated gB2. Temperature shift experiments indicated that retrocyclin 2 and human alpha defensins human neutrophil peptide 1 (HNP 1) to HNP 3 protected human cells from HSV-2 by different mechanisms. Retrocyclin 2 blocked viral attachment, and its addition during the binding or penetration phases of HSV-2 infection markedly diminished nuclear translocation of VP16 and expression of ICP4. In contrast, HNPs 1 to 3 had little effect on binding but reduced both VP16 transport and ICP4 expression if added during the postbinding (penetration) period. We recently reported that theta defensins are miniature lectins that bind gp120 of human immunodeficiency virus type 1 (HIV-1) with high affinity and inhibit the entry of R5 and X4 isolates of HIV-1. Given its small size (18 residues), minimal cytotoxicity, lack of activity against vaginal lactobacilli, and effectiveness against both HSV-2 and HIV-1, retrocyclin 2 provides an intriguing prototype for future topical microbicide development.

An anti-apoptotic role for galectin-3 in diffuse large B-cell lymphomas.

Increased resistance to apoptosis promotes lymphomagenesis with aberrant expression of cell survival proteins such as BCL-2 and c-MYC occurring in distinct lymphoma subtypes. Galectin-3 is an anti-apoptotic protein that protects T cells, macrophages, and breast carcinoma cells from death triggered by a variety of agents. We have found high levels of galectin-3 protein expression in a subset of B-cell neoplasms including diffuse large B-cell lymphoma (DLBCL), primary effusion lymphoma (PEL), and multiple myeloma (MM), in both cell lines and patient samples. However, we failed to detect galectin-3 in Burkitt lymphoma (BL), follicular lymphoma (FL), marginal zone lymphoma (MZL), MALT lymphoma or B-small lymphocytic lymphoma (B-SLL) cell lines or patient samples. To determine whether galectin-3 expression protects B cells from apoptosis, galectin-3-negative BL cells were transfected with a galectin-3 expressing plasmid, which resulted in markedly increased resistance to anti-Fas-induced cell death. In contrast, galectin-3-positive PEL cells transfected with an amino-terminal truncated galectin-3 vector showed increased sensitivity to anti-Fas induced apoptosis. During normal B-cell development, galectin-3 expression was lowest in germinal center and plasma B cells, from which DLBCL, PEL, and MM derive, and highest in long-lived naïve and memory B cells. This pattern of expression suggests that aberrantly increased galectin-3 levels in specific B-cell populations may yield a protective advantage during transformation and/or progression of certain B-cell neoplasms.

Examination of Chlamydia trachomatis infection in environments mimicking normal and abnormal vaginal pH.

BACKGROUND: It has long been assumed that a healthy acidic vaginal environment inhibits infection by Chlamydia trachomatis. The research objectives were to evaluate the effect of pH on C trachomatis infection by two in vitro methods, to assess pH effect at different serial dilutions of C trachomatis elementary bodies (EBs), and to examine protection by an antibiotic peptide, protegrin (PG-1), over a pH range. GOALS: The goals of this study were to test the hypothesis that acidic pH inhibits C trachomatis infection and to determine the ability of PG-1 to provide protection at acidic and neutral pH. STUDY DESIGN: The effect of pH on C trachomatis was examined using two pH-adjusted preincubation shell vial assays. C trachomatis EBs (serovars L2, D, and E) were exposed to pH-adjusted media, with and without PG-1, and infection was assessed by inclusion forming unit (IFU) formation in McCoy cell monolayers. RESULTS: Acidic pH in preincubation media markedly decreased IFUs by both in vitro methods. Serial dilution experiments showed a 3- to 10-fold reduction in IFUs for C trachomatis (L2 and E) at pH 5.0, compared with those at pH 7.5. C trachomatis (D) showed a 17- to 23-fold reduction in IFUs (serial dilutions 1:1-1:4). PG-1 protected McCoy cell monolayers from infection by C trachomatis after exposure to varied pH environments. CONCLUSION: Acidic pH exposure significantly reduced C trachomatis infection in vitro. Our results support the hypothesis that a healthy acidic vaginal environment protects women from C trachomatis infection. In addition, antibiotic peptides may provide protection as topical microbicides, regardless of vaginal pH.